Every time you flush a disposable wet wipe down the toilet, just imagine sewer workers screaming, “Nooooo!” That’s because no one wants to battle a monster “fatberg” of the sort currently blocking London’s sewers.

If you thought a fatberg the size of a 747 was impressive, you ain’t seen nothing yet: Thames Water workers have embarked on a three-week “sewer war” against a massive, hardened chunk — made of wet wipes, fat, condoms, diapers, and sanitary products — that’s...